This invention relates to an imaging device adopted for photo-printing system such as photoprinters and photocopy machines, and more particularly, an imaging device for fixing an image by applying a pressure on a photo and pressure sensitive recording sheet on which a latent image is formed by exposing a light thereon.
The photo and pressure sensitive recording sheet generally comprises, for instance as disclosed in the U.S. Pat. No. 4,440,846, microcapsules and chromogenic materials laid one upon another on a substrate such as a paper. The hardness of microcapsules increases when exposed to light while the chromogenic materials are colourized when mixed with a substance enveloped in microcapsules. Accordingly, in order to print an image on this type recording sheet, first a latent image is formed by exposing a light thereon to harden the microcapsules in the area on the sheet corresponding to the negative image of an original manuscript, and thereafter the latent image is developed by applying a pressure thereon to rupture the unhardened microcapsules in the area where the light is exposed. With this pressure application, the chromogenic materials on the recording sheet in the area where the light is not exposed are colourized as they react with the substance streamed out of the ruptured microcapsules. Thus, the positive image is fixedly printed on the recording sheet. Generally, with the imaging device utilizing the photo and pressure sensitive recording sheet, the developement is carried out by passing the sheet between the pair of pressing rollers.
The deepness level of colourization of chromogenic materials is influenced by the hardness of microcapsules and the hardness thereof depends upon the degree of exposure, i.e., the exposure time and the luminous flux. On the contrary, when the degree of exposure is fixed, the deepness of colourization of chromogenic materials is influenced by the degree of pressing, i.e., the pressing time and the pressure. FIG. 8 is a gragh showing the characteristic curve of the general relatioship between the pressure (kg) and the colourization level (%) under the condition that the degree of exposure and the pressing time are fixed. The inclination angle of the characteristic curve decreases when the degree of exposure is increased or when the pressing time is decreased. In other words, the deepness of colourization depends upon the degree of exposure and/or the degree of pressing.
In the prior devices, however, the pressure applied by the pressing rollers is predetermined depending upon the thickness of the recording sheet and so on. Accordingly, when the degree of exposure for forming the latent image on the sheet is varied by, for instance, the changing with time of the condition of a light source, which results in excess or shortrage of exposure and the hardening level of microcapsules is varied. But, as the pressure applied by the pressing rollers is always constant as above described in the prior devices, the deepness level of the colourization of the chromogenic materials on the recording sheet varies and is not uniform, thus it becomes impossible to obtain the visible image of the stable colour deepness.
Moreover, a linear-shape light source is generally utilized when exposing a light on the recording sheet and, as illustrated in FIG. 9 which shows a luminous flux distribution of the linear-shape light source, the luminous flux is the highest at the lateral center of the linear-shape light source and gradually decreases toward the ends thereof. In general, the effective light-emission length of the linear-shape light source is the length wherein the luminous flux does not decrease more than 20 percents, and therefore, both end portions of the linear-shape light source are not utilized for exposing a light on the recording sheet in the photo-printing system. However, even within the effective light-emission area of the linear-shape light source, there are differences in the luminous flux so that the hardeness level of light exposed microcapsules differs depending upon their positions relative to the linear-shape light source. Accordingly, when the degree of the pressing applied thereto is substantially same, the deepness level of colourizing becomes uneven depending upon the positions, thus the colour deepness of the image printed becomes uneven.